This section is intended to provide background information to facilitate a better understanding of the various aspects of the described embodiments. Accordingly, it should be understood that these statements are to be read in this light and not as admissions of prior art.
Blowout preventers (BOPs) are used extensively throughout the oil and gas industry. Typical BOPs are used as a large specialized valve or similar mechanical device that seal, control, and monitor oil and gas wells. The two categories of BOPs that are most prevalent are ram BOPs and annular BOPs. Blowout preventer stacks frequently utilize both types of BOPs, typically with at least one annular BOP stacked above several ram BOPs. The ram assemblies (i.e., rams) in ram BOPs allow for shearing drill pipe in the case of shear ram assemblies, sealing off around drill pipe in the case of pipe ram assemblies or variable bore ram assemblies, or sealing off the bore in the case of blind ram assemblies. Typically, a BOP stack may be secured to a wellhead and may provide a safe means for sealing the well in the event of a system failure.
A typical ram BOP includes a main body or housing with a vertical bore. Ram bonnet assemblies may be bolted to opposing sides of the main body using a number of high tensile fasteners, such as bolts or studs. These fasteners are required to hold the bonnet in position to enable the sealing arrangements to work effectively. One or more elastomeric sealing elements or “packers” may then be used to form a seal within the main body and against the ram assemblies. There are several configurations, but essentially they are all directed to preventing a leakage bypass between the mating faces of the ram assembly and the main body. Each bonnet assembly includes a piston that is laterally movable within a ram cavity of the bonnet assembly by pressurized hydraulic fluid acting on one side of the piston. The opposite side of each piston has a connecting rod attached thereto that in turn has a ram assembly mounted thereon.
The ram assemblies are designed to move laterally toward the vertical bore of the BOP to shear or seal off on any object located therein. For a shear ram BOP, the shear ram assemblies are used to shear or cut any object located in the vertical bore of the blowout preventer. Pipe ram assemblies and variable bore ram assemblies utilize seals or packers that close in on and seal off on a tubular within the vertical bore of the BOP, such as a section of drill pipe used during drilling operations. Blind ram assemblies also utilize seals, in which the ram assemblies close in and across the bore of the BOP when no object is present to seal across the bore.
The annular BOP utilizes an annular or a hemispherical piece of rubber usually reinforced with inserts. Unlike a ram-type BOP, which closes with a horizontal motion, an annular BOP closes inward around the drill string in a smooth simultaneous upward and inward motion to seal on the pipe or the open hole. The geometry of this movement reduces internal stresses and friction between the BOP body and the sealing element, which translates into a longer field life with less maintenance. The annular design may also operate with a much lower operating pressure, reducing the number of hydraulic accumulators necessary, and thereby reducing cost and complexity of the BOP.
Packers used for either a ram-type or annular BOPs can be designed to seal around pipe of a specific size in the BOP bore when the BOP is closed. Other packers though may be configured to seal around a range of pipe sizes, and are referred to as variable bore packers. Both packers form a pressure tight seal during a kick until the well bore pressure can be controlled.
To form a proper seal with a variable bore BOP, the packer material must be of a low enough durometer to close against the pipe and provide enough pressure for a range of pipe diameters. However, a low durometer also tends to make the packer suffer from lack of support during the loading process. Increased durometer packers may be used but more force is needed to form a seal, sometimes resulting in an inadequate seal due to incomplete closure and/or low pressure. Additionally, the increased durometer packers may not be able to seal against as wide a range of pipe diameters. With both the ram-type and annular variable bore BOPs, the packer typically includes an annular or two semi-circular elastomeric sealing elements with an array of support inserts embedded in the elastomeric material. The inserts are molded within the elastomeric material in a pattern around the opening of the elastomeric material, forming unitary structure. The structure allows the plurality of packer inserts to move and seat against different diameter tubular members and also helps prevent extrusion of the elastomeric material between the packer inserts and the tubular member.
Even with inserts, however, some packers still have durability issues. Although inserts help prevent extrusion, the inserts still include gaps between the insert tips and the pipe when the packer is actuated. When subject to load, the packer elastomeric material may still extrude through these small gaps, causing the packer material to tear and break apart and thus lose ability to form an adequate seal.